Line In Jack

First, do not short the tip and ring.  The LineIn input is a pseudo balanced input. Shorting tip to ring is shorting LineIn+ to LineIn-  If you only need to use two wires, use tip (+) and ring (-) and leave the ground disconnected.  If you have 3-wires, then it is tip (+), ring (-) and ground (cable shield) and do not connect anything together.  The input is converted to unbalanced at the codec input.

Second, why are you using a SignalLink instead of DAX (unless you are operating remote)?

DAX performs a bit perfect transfer of audio data between the radio and the digital mode application, whereas a SingnalLink (or any other sound card) must do an analog to digital back to analog conversion which degrades the signal quality. 

Also, the ADC/DAC in the SignalLink is not as high a quality as what is in the FLEX-6000, so in addition to signal degradation due to an unnecessary analog to digital back to analog conversion, the ADC/DAC is also degrading your signal to noise characteristics.

And the analog cabling between the radio/Maestro and the SignalLink increases the chances of getting RFI into the connected device whereas DAX uses no physical cables.

Unless you are connecting something like a Pactor modem (hardware) to the radio that requires using the analog audio inputs and outputs, no one operates digital mode applications using the FLEX-6000 using analog inputs unless you are trying to operate remote using the Maestro, which if that is the case, I can understand why you are doing it this way